Electrophotographic printing element



June 6, 1961 J. G. JARVIS 2,987,395

ELECTROPHOTOGRAPHIC PRINTING ELEMENT Filed D60. 25, 1956 DYE- SENS/T/ZED Zl/VC OX/DE-RES/N Zl/VC OXIDE RES/IV SUPPORT BY fg/ZIVENTOR.

ATTORNEY 8 AGE/VT United States Patent it) 2,987,395 ELECTROPHOTOGRAPHIC PRINTING ELEMENT James G. Jarvis, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Dec. 26, 1956, Ser. No. 630,480 4 Claims. (Cl. 96-1) This invention relates to an electrophotographic printing element for use in a process of electrostatic printing.

A well-known electrostatic printing process involves giving an element carrying a photoconductive layer a blanket electrostatic charge under subdued illumination or in the dark, for example, by ion transfer from a corona discharge followed by exposure to light modulated by an image, for example, by projection of a photographic image thereon with the result that the illuminated areas of the photoconductive layer are discharged leaving an electrostatic image on the layer. The resultant electrostatic image is then dusted with an electroscopic powder such as a pigmented resin powder carrying an opposite electrostatic charge with the result that the powder adheres to the electrostatically charged image. The resulting image may be used in a number of ways; for example, a portion of the powdered image may then be transferred imagewise to a paper support and fused thereon to provide a very sharp reproduction of the original subject.

An electrophotographic printing process recently described by Young and Greig, RCA Review, vol. 15, No. 4, December 1954, pp. 46984, utilizes as the electrophotographic element ordinary paper carrying a layer of zinc oxide primarily sensitive to the ultraviolet region of the spectrum, that is, an electrostatic charge on the layer can be discharged by Wavelengths of light in the ultraviolet region. However, the zinc oxide layer may be optically sensitized with organic dyes to extend the sensitivity throughout the visible spectrum. The result is to appreciably increase sensitivity, for example, to tungsten light. Some of the dyes which are effective sensitizers for this purpose are Acridine Orange, Fluorescein, Eosin, Methylene Blue and Rose Bengal.

In sensitizing the zinc oxide layers, the usual method has been to merely add the dye to the coating mixture in suitable quantity, and as a result objectionable coloration is imparted to the zinc oxide layer prepared therefrom.

I have discovered a method for preparing photoconductive zinc oxide layers which includes first applying to a paper or other support a photoconductive layer composed of zinc oxide, such as ordinary white zinc oxide dispersed in a resin, and coating thereover a second thin resinous photoconductive layer of zinc oxide which has been sensitized, for example, by adding a sensitizing dye or which layer is inherently more sensitive or has a different sensitivity than the underlying layer of zinc oxide. One result is that so much less sensitizing dye is required for maximum light-sensitivity than is required when the dye is dispersed throughout a single layer of zinc oxide, that the coating has comparatively little color. Also, the decreased performance at high humidities usually accompanying dye sensitization is not observed when the sensitizer is present only in the surface layer. Furthermore, a substantial speed gain is obtained partly due to radiations being more effectively reflected back into the sensitized outer stratum from the underlying light-colored unsensitized layer. Similarly, great improvement in the dark decay rate is effected by the two-layer coating technique.

Effects of this type have not been observed in the art of photographic silver halide emulsions inasmuch as optical sensitizing dyes for silver halide ordinarily require uniform dispersion in the silver halide emulsion in order to obtain maximum sensitivity.

One object of the invention, therefore, is to provide electrophotographic elements having increased light sensitivity and low visual color. Other objects of the invention will become apparent from consideration of the following description of the invention.

These objects are accomplished in part by coating a support such as paper or metal, for example, zinc and aluminum supports, with a resinous photoconductive layer of zinc oxide not especially sensitized or free of optical sensitizing dye, and coating thereover a second :layer of resinous photoconductive material containing sensitized zinc oxide. The sensitization of the latter layer may be an inherent property, for example, of the so-called pink type of zinc oxide mentioned hereinafter, or the sensitization may have been imparted to the zinc oxide by addition of a sensitizing dye such as mentioned above. The sensitization of the outer layer may therefore take the form of a chemical type of sensitization where the level of sensitivity to light of a given wavelength has been increased, or the sensitization may take the form of optical sensitization where an agent such as a dye has extended the light sensitivity to regions of the spectrum in addition to the ultraviolet region.

The accompanying drawing illustrates in a greatly enlarged cross-sectional view the appearance of a typical electrophotographic element of the invention.

In the drawing, layer 10 of the element represents the support such as paper, layer 11 a photoconductive layer of a mixture of zinc oxide and a resin, and layer 12 a thin layer similar to layer 11 except being sensitized with, for example, a dye capable of extending the sensitivity of zinc oxide beyond the ultraviolet region.

The improved methods of making the electrophotographic elements of my invention are illustrated in the following examples.

Example 1 parts of zinc oxide were mixed with 20 parts of a styrene-butadiene resin and sufficient toluene to obtain a solid concentration of 10 percent. 0.02 percent of Rose Bengal (based on the amount of zinc oxide present) was added. An ordinary barytacoated photographic paper base was then coated with this composition and it was ascertained that approximately 1.5 grams of the dried coating per square foot were required to obtain optimum sensitivity and to be capable of holding enough charge to attract a maximum deposit of powder in the unexposed areas, whereas only 0.5 gram per square foot were required to obtain optimum sensitivity when the composition was coated on a similar paper base which had previously been coated with about 1 to 2 grams per square foot of an identical unsensitized resinous zinc oxide composition. The result was that the double-coated element was about 0.2 log E faster (about 59% faster) than the single-coated element and had considerably lower visual color, which color was barely perceptible visually. Paper coated by this method can be expected to have a reflectance at wavelength of maximum absorption (560 millimicrons) of about 56 percent compared to the unsensitized paper sheet of 82 percent reflectance.

Example 2 A coating composition was prepared as in Example 1 containing zinc oxide, toluene and the polymer. The following amounts of dyes were added to the dope:

Percent Anhydro 3,3 di-B-carboxyethyl-S,5-dichloro-9- ethylthiacarbocyanine hydroxide .0087

Percent 3 carboxymethyl-S-(3-ethyl-2(3)-benzothiazolylidene)rhodanine .027 3,3-di-[3-hydroxyethylthiadicarbocyanine bromide .0087

The resulting dope was coated at a coverage of 0.6 gram per square foot upon a paper which has previously been coated with the same zinc oxide-resin composition unsensitized. The resulting coating had a barely perceptible coloration, and it was found that when it was electrostatically charged as described, the paper was sensitive enough to be capable of being selectively discharged with red, green and blue light filtered exposures from a photographic color transparency.

Useful results are obtained when the sensitized zinc oxide layer contains about 0.1 to 1.0 gram of zinc oxide per square foot of surface and from about 0.001 to 0.5% by weight of optical sensitizing dye (based upon the zinc oxide present).

The photoconductive elements prepared as illustrated in the examples above can be used as described in the above RCA article, a suitable method for developing the electrostatic images being to apply a ferromagnetic powder thereto by means of a magnetic brush. The resinous component of the photoconductive layers of the elements of the invention include a large number of polymers having fairly high dielectric strength and which are good electrically insulating film-forming vehicles. These materials include such resins as sold under trade names such as Plaskon ST 856, Rezyl 405-18, Plyolite S7, Styresol 4440, DC-804, SR82, etc. These resins comprise styrene-butadiene copolymers, silicone resins, styrene-alkyd resins, silicone-alkyd resins, soyaalkyd resins, polyvinyl chloride, polyvinyl acetate, etc. The methods of making such resins have been previously described in the prior art, for example, styrene-alkyd resins can be prepared according to the method described in US. Patent 2,361,019, October 24, 1944; US. Patent 2,258,423, October 7, 1941; US. Patent 2,453,665, November 9, 1948; etc. Other binders such as parafi'in, mineral waxes, etc. can also be employed. These binders are generally characterized as having marked hydrophobic properties, that is, are substantially free of watersolubilizing groups such as hydroxyl, free acid groups, amide groups, etc., and as being good electrical insulators or having high electrical resistivity. These binders can be easily dissolved in organic solvents having a boiling point below the charring temperature of a paper support. Also, these. binders have the desirable property of readily dispersing the photoconductive zinc oxide. Some resinous binders will be found to be not as good insulators as would be desired and therefore electrostatically charged layers containing such binders cannot be stored as long as would be desired without losing the charge. In some instances it is advantageous to use a different resin in the underlying photoconductive layer than in the thin outer layer. For example, by using a styrene-butadiene polymer only in the outer stratum favorable properties of the polymer such as humidity resistance and toughness can be taken advantage of while the tendency of the polymer to yellow is minimized.

Zinc oxide which is the preferred photoconductive constituent of the elements of the invention should have a relatively small particle size of the order of less than 0.5 micron mean diameter. Suitable zinc oxides are readily obtainable and can be purchased under a variety of trade names such as White Seal No. 7 or Green Seal No. 8 (New Jersey Zinc Company). Suflicient resinous binder should be employed with the photoconductive materials to cover each of the particles and to isolate it from the surrounding particles in the composition. The most useful or optimum ratio of photoconductive material to binder for a particular binder can be readily determined by making a series of test coatings wherein the quantity 4 and relative amounts of photoconductive material to hinder are employed. If desired, Zinc oxide prepared by the methods of US. Patents 2,727,807 and 2,727,808 may be used in the elements of the invention. It has been found that different types of Zinc oxides can be used advantageously in the respective strata of the electrophotographic elements in the manner of the above examples. For example, the conventional white type of zinc oxide may be employed in the lower stratum, and the pink type (US. Patents 2,727,807, 2,727,808) having inherent sensitivity extended to the red region of the spectrum, in the outer stratum. As described in the mentioned patents, one way of preparing the panchromatic colored type of zinc oxide is to heat white zinc oxide with carbon dioxide and ammonia gas. Electrostatic surface charges on such elements can be destroyed locally by Wavelengths of light to which the white variety of zinc oxide is insensitive. Such coatings are 2 to 3 times faster than coatings composed entirely of the white Zinc oxide. In the manner of the examples, sensitizing dye may also be contained in the outer layer of pink zinc oxide, and little or none in the underlying layer.

The above-mentioned dyes are illustrative of the many sensitizing dyes which can be used in the photoconductive elements in the manner described in the examples.

What I claim is:

1. An electrophotographic element comprising a support having thereon a first stratum of substantially colorless photoconductive zinc oxide dispersed in a resinous insulating binder and superimposed thereon and contiguous thereto a second discrete stratum of (1) from about 0.1 to 1.0 gram photoconductive zinc oxide per square foot and (2) from about 0.001 to 0.5% based on the zinc oxide present in said second stratum of a dye extending the spectrial sensitivity of the zinc oxide into the visible spectrum dispersed in a resinous insulating binder, said second stratum being thinner than said first stratum, the sensitizing dye in said element being confined to said second stratum.

2. The element of claim 1 wherein the support is paper.

3. A process for the preparation of an electrophotographic element which comprises coating upon a support a first stratum of a substantially colorless photoconductive zinc oxide dispersed in resinous insulating binder, and coating directly on said stratum a second stratum of (1) from about 0.1 to 1.0 gram photoconductive zinc oxide per square foot and (2) from about 0.001 to 0.5% based upon the zinc oxide present in said second stratum, of a dye extending the spectral sensitivity of the zinc oxide into the visible spectrum dispersed in a resinous insulating binder, said second stratum being thinner than said first stratum, the sensitizing dye in said element being confined to said second stratum.

4. The process of claim 3 wherein the support is paper.

References Cited in the file of this patent UNITED STATES PATENTS 2,803,541 Paris Aug. 20, 1957 FOREIGN PATENTS 201,416 Australia Apr. 13, 1956 OTHER REFERENCES Young et al.: R.C.A. Review, December 1954, pp. 469-484.

Mattiello: Protective and Decorative Coatings, vol. 11, page 375, John Wiley & Sons, Inc., N.Y., 1942.

Supplement 9 to Raw Materials Indes, National Paint, Varnish and Lacquer Association, Inc., Washington, D.C., May 1949, pp. 39 and 40.

Amick: R.C.A. Review, vol. 20 (No. 4), December 1959, pp. 770-784. 

1. AN ELECTROPHOTOGRAPHIC ELEMENT COMPRISING A SUPPORT HAVING THEREON A FIRST STRATUM OF SUBSTANTIALLY COLORLESS PHOTOCONDUCTIVE ZINC OXIDE DISPERSED IN A RESIOUS INSULATING BINDER AND SUPERIMPOSED THEREON AND CONTIGUOUS THERETO A SECOND DISCRETE STRATUM OF (1) FROM ABOUT 0.1 TO 1.0 GRAM PHOTOCONDUCTIVE ZINC OXIDE PER SQUARE FOOT AND (2) FROM ABOUT 0.001 TO 0.5% BASED ON THE ZINC OXIDE PRESENT IN SAID SECOND STRATUM OF A DYE EX- 